1. Field of the Invention
The present invention relates to film devices provided with resistive elements thereon, and in particular, relates to a film device provided with a resistance-adjustable resistive element in which the printed resistive element has a slight variation in resistance.
2. Description of the Related Art
Film device provided with resistance-adjustable resistive elements have been used in control panels for portable telephones, video cameras and the like, and in membrane switches for operating electrical equipment, e.g. washing machines.
In a typical conventional film device provided with a resistan for control panels, as shown in the plan view in FIG. 5, a conductive circuit pattern 2 is printed on a base film 1 composed of a polyester film or the like using a silver paste or the like, and the terminal of the conductive circuit pattern 2 forms a terminal section 2c which communicates to external devices. The conductive circuit pattern 2 is provided with a plurality of key switch sections 3, corresponding to, for example, buttons of the portable telephone, each key switch section comprising a pair of contact points 2a and 2b arranged close to each other. Also, the conductive circuit pattern 2 is provided with a plurality of chip electrical parts 4, e.g. LED, each chip electrical part bridging a pair of terminal sections 2f. A resistive element 5 having a resistance of approximately 300 .OMEGA. is printed using a carbon paste or the like adjacent to each LED 4. The LED 4 is therefore connected to the terminal section 2c through the resistive element 5 which controls the current flow in the LED 4.
A movable contact member composed of a dome-shaped metal blade spring is put on each of key switch sections 3, shown by a broken circle in FIG. 5. When pressing the movable contact member, the two contact points 2a and 2b are connected or disconnected to each other through the movable contact member for switching operation.
In the operation of the above-mentioned film device provided with a resistance-adjustable resistive element, as shown in the circuit diagram in FIG. 6, a voltage, e.g. 5 volts, is applied to the terminal section 2c, which is connected to a terminal section 2a of the conductive circuit pattern 2, through a pull-up resistor not shown in the drawing. When the movable contact member of the key switch section 3 is pressed to connect the two contact points 2a and 2b, the signal of the key switch section 3 is obtained as a change in a voltage level (from a high level to a low level) at the terminal section 2c connected to the contact point 2a. Further, a constant voltage, e.g. 5 volts, is applied to all the series circuits, each composed of the resistive element 5 and the LED 4, to uniformly illuminate all the key switch sections 3.
The above-mentioned film device provided with a resistance-adjustable resistive element for membrane switches has, as shown in FIGS. 7 and 8, a configuration in which a lower electrode sheet 11 is overlaid with an upper electrode sheet 12 separated by a spacer film 13. The lower electrode sheet 11 comprises a base film composed of a polyester film or the like and a given conductive circuit pattern 2 printed thereon using a silver paste or the like. A portion of the conductive circuit pattern 2 consists of a terminal section 2c and a plurality of lower contact points 2a, and is provided with a plurality of chip electrical parts, e.g. LEDs 4, and a plurality of resistive elements 5 formed from a carbon paste or the like, in which each LED and each resistive element are connected to the constituent of the conductive circuit pattern 2 in series.
The spacer film 13 is composed of a polyester film or the like and is provided with a plurality of openings 13a at positions which correspond to the lower contact points 2a and LEDs 4 on the lower electrode sheet 11.
The upper electrode sheet 12 is also formed by printing a conductive circuit pattern 2 and the upper contact point 2b on a flexible base film composed of a polyester film or the like using a silver paste or the like, as in the lower electrode sheet 11.
When the film device provided with a resistance-adjustable resistive element is used in severe environments, e.g. washing machines, a protective layer (not shown in the drawing) formed from, for example, carbon ink is provided on the conductive circuit patterns 2 of the upper and lower electrode sheets 12 and 11 excluding the connecting section of each LED 4 and each resistive element 5 in order to prevent circuiting between the conductive patterns 2 due to silver migration.
The upper and lower electrode sheets 12 and 11 are laminated through the spacer film 13 such that the contact points 2b and 2a and the LED 4 are positioned in their respective openings 13a of the spacer film 13. A film device provided with a resistance-adjustable resistive element for membrane switches provided with a plurality of key switch sections 3 at the contact points 2b and 2a is manufactured in such a manner.
Before use of the film device provided with a resistance-adjustable resistive element, the lower electrode sheet 11 is adhered onto a rigid substrate such as steel sheet in order to maintain its flatness, whereas the upper electrode sheet 12 is covered with a flexible, designed surface sheet 15 which forms an operation surface.
In operation of the film device provided with a resistance-adjustable resistive element, when pressing a given key switch section 3 of the upper electrode sheet 12, the upper electrode sheet 12 is bent and the upper and lower contact points 2b and 2a at the opening 13 of the spacer film 13 are switched, i.e., connected or disconnected. A constant voltage is applied to all the LEDs 4 to uniformly illuminate all of the key switch section 3 due to light emission from the LEDs. The resistive element 5 restricts the current flow in the LED 4.
The resistive element 5 is formed by printing in all the conventional film device provided with resistance-adjustable resistive elements for control panels and membrane switches. In printing methods, the resistance of the resistive element varies due to variations in the resistance and the thickness of the printed paste, such as a carbon black paste. Although the resistive elements have relatively stable resistances in the same production lot, i.e., variances of .+-.20%, but has very large variances of .+-.60% between different lots.
As a result, the brightness of the LEDs 4 varies when the conventional resistive elements 5 are used for controlling the current flowing in the LEDs 4.
All the resistive elements 5 must therefore be inspected to check whether these products satisfy a predetermined resistance range in the production process. Failed products having resistances out of the range cause an increase in cost due to a low yield because the failed products are discarded.